1. Field of the Invention
The present invention relates to safety closures for containers and more particularly to closures which release under adverse conditions to vent the containers. Such release can be under conditions of excessive temperature in which a portion of the cap is melted to open the normally closed aperture to the container interior. In certain constructions the buildup of internal pressures is effective to open the vent passage or to distort the cap as an indication of the buildup.
2. Description of the Prior Art
Traditionally, storage tanks and the like have been fitted with a pipe on their upper surface which was sealed with a fusible metal cap. These caps made of metal were almost exclusively of brass, aluminum or malleable stainless. These caps were constructed in such a fashion as to breach a seal system by melting out a fusible link which sealingly retained a metal closure for the aperture in a metal ferrule upon the attainment of a certain predetermined temperature of the vessel in which the material is stored, and thus vent the warm high pressure volatile internal material to the ambient external atmosphere. This method circumvented the possibly explosive buildup of internal vapor pressure of these volatile and usually flammable materials.
These metal cap safety devices presented a number of major practical disadvantages. First, should one of these metal caps be dropped inadvertently, there was a high probability that the impact would so deform the threaded areas that the cap would be functionally useless. Second, the extreme mass of such caps, up to six times the mass of the hereinafter described device, caused great inconvenience in transport with a working vehicle such as a tractor trailer rig. Third, these caps are often used in an environment rich in the vapors of the transported material as well as being exposed to all-weather conditions. This combination of corrosive forces often caused a substantial corrosion of the threads of the cap and its subsequent fusion or "locking" on the threads of the metal fitting of the storage unit. This caused major expense in the forced removal of the cap and the subsequent damage to the threads of the storage unit emergency vent pipe, which was permanently anchored to the storage unit. Fourth, the metal fittings generally had to be lubricated to facilitate easy operation of the cap threads on the pipe threads. In a normal use such lubricants as greases and oils are of no concern but in vessels containing highly purified chemicals these oils and greases may infiltrate the material being transported and substantially damage the quality and usefulness of the material. Individually, these problems are of import but when combined they form a substantial detriment to the safe economical use of these metal caps.
Also, if a metal cap does maintain its functional integrity there are two further problems present. First, petro chemical transport and storage units are periodically cleaned on their interior surfaces with high pressure steam. The fusible caps are usually removed from the tank pipe and placed on the adjacent tank surface. As the steam cleans the interior of the tank it heats the tank. This elevation in temperature is transmitted from the tank surface to the cap fusing mechanism by the metallic body of the metal cap, which has a high thermal conductivity. Frequently the fuses are triggered in this fashion rendering them useless. These fuses must be shipped back to the factory for refusing which is not only time consuming, but also fairly expensive, when compared to the purchase price of an entire new fuse cap.
An object of the present invention is to improve sealable safety vent release caps.
Another object is to enable release caps to be reconstructed simply in the field.
A third object is to eliminate the need for seal gaskets for safety release caps.
A fourth object is to avoid accidental fusing of the fusible elements of safety release caps.
A further object is to combine thermally fusible venting with pressure sensitive venting in a replaceable cap.